This invention relates to a novel torsion socket for use in tightening wheel nuts for automobiles, lorries and the like. More specifically, this invention relates to a torsion socket for use with an impact wrench to limit the tightening torque applied to wheel nuts, and the like, during secondary or after market changing or rotation of tires or repairing brakes where a tire must be removed and remounted.
The design specification with respect to torque to be applied to a lug nut for original equipment can vary significantly; depending on the size and design of a vehicle. During initial installation, rather elaborate machines run-up nuts to design tolerances. However, in an after market environment elaborate units are not practical and are replaced by a standard power-operated hand tool or impact wrench and a variety of socket sizes.
Various types of sockets have been used in the past with impact wrenches to facilitate mounting a wheel on an axle. Typically, one end of a socket is directly mounted to the drive shaft of an impact wrench and the other end of the socket engages a wheel nut. During operation, an impact wrench, acting through an internal clutch, delivers rapid turning blows to the wheel nut to tighten the wheel nut about a threaded hub bolt. Each blow of the impact wrench partially rotates the wheel nut so that the nut is rapidly tightened in increments with rotational force at impact being at a maximum and then rapidly decaying to zero until the next blow.
With conventional impact wrenches and hand connected sockets, an operator attempts to determine when the wheel nut has been adequately tightened by listening to the sound of the impact wrench and slippage of the impact wrench internal clutch. In this connection at least some believe that a skilled operator can audibly distinguish the sound of a wheel nut being rotated from the sound of the wheel nut resisting rotation. However, by the time an operator can audibly detect the sound of clutch slippage, torque on a wheel nut has already exceeded engineering specifications.
Although impact wrenches are fairly simple to operate, a conventional impact wrench gives a high torque to a hard joint such as a wheel nut. The high impact force delivered to a wheel nut using a standard impact socket, combined with an operator's inability to visually or audibly determine when manufactures design specification for torque has achieved usual results in overtightening the nut. Since modern car wheel rims may be with rather thin elements, it is possible to deform the tire rim by overtightening a wheel nut. In some instances, overtightening a wheel nut may even shear an associated hub bolt.
In addition to the above mentioned problems, overtightening a wheel nut can misalign a wheel with respect to an axle because the overtightened portion of the wheel hub will be canted inward relative to the axle. Misalignment of a wheel can impede the performance of the vehicle and result in rapid tire wear and/or poor steering control. Still further over tightening can adversely affect brake pad wear on disc brake systems.
Another significant problem caused by overtightened wheel nuts is that, as a tightening force is applied to the wheel nut, the associated hub bolt absorbs some of the extra impact force and may transfer the force to points of contact with the wheel hub. In some instances over tightening can result in deforming the bolt-receiving holes in a wheel. If the bolt-receiving holes are deformed, the wheel cannot be exactly centered about the axle which, again, decreases vehicle performance.
In order to reduce the torsion force delivered to the wheel nut, torque sockets have been introduced. Torque sockets are made of metals having elastic properties so that excess torsion force is distributed along the socket itself and not delivered to the wheel nut. The stem or waistline of the torque socket acts as a torsion spring and absorbs the excess power of an impact wrench. Once a wheel nut is adequately tightened, a driver end of the torque socket continues to turn with each blow of the impact wrench, while the nut end stops rotating. The elastic properties of the torque socket enables the drive end to spring back between each blow and/or store rotation as spring energy. An example of a prior torque limiting spanner is disclosed in U.K. patent No. 1 521 461 of common assignment With the instant application.
Although torque sockets of the type described above provide a marked improvement over hard drive systems and have achieved considerable commercial attention, previously known torque sockets may not always sufficiently absorb rapidly repeated blows of an impact wrench. Since a user cannot visually determine when nut tightening is complete, blows which are not adequately absorbed by the torsion socket may be transmitted to the nut and damage the associated wheel bolt, tire hub, or rim. As stated in the preceding, these problems can become exacerbated as more sophisticated, thinner wheels are utilized. Still further, torsion strength and structural integrity of previously known torsion sockets is relevant when attempting to accommodate elongated or domed lug nuts.
The difficulties suggested in the preceding are not intended to be exhaustive but rather are among many which may tend to reduce user satisfaction with prior torque sockets. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that torsion sockets appearing in the past will admit to worthwhile improvement.